Geraldine A. Ferraro drew her biggest and most exuberant crowd of the campaign today as more than 25,000 wildly cheering supporters flooded the University of Massachusetts campus to hear the Democratic vice-presidential nominee blast President Reagan's record on arms control.

Ferraro was particularly strident in condemning the administration's Strategic Defense Initiative, popularly known as "star wars." Even if the proposed defense against Soviet missiles is 99.7 percent effective, she said, 25 nuclear warheads still would "leak" through to devastate American cities.

"In my book, that's not much of a defense. Remember: If we extend the arms race into space, the Soviets will follow. Let's say enough is enough. The sky's the limit," she added.

On a balmy and blue Indian Summer day, Ferraro demonstrated her drawing power among young voters despite some of her own concerns, voiced last week at the University of Washington, that "young people are moving toward the Republican ticket in great numbers."

The crowd, which blanketed the sides of a natural ampitheater and even clustered in the branches of trees more than 200 yards from the speakers' platform, interrupted Ferraro dozens of times with applause or chants of "Ger-ry!"

In a theme likely to foreshadow part of Walter F. Mondale's debate agenda Sunday night, Ferraro condemned a recently publicized CIA primer on assassination in Central America as "written more in the spirit of Stalin than Jefferson."

"It's an insult to the American people. Moreover, anyone who believes for one minute that refining the murder techniques of Central Americans advances our national interests is gravely mistaken," she added.

Ferraro also recited a remark by Peter Teeley, Vice President Bush's press secretary, who recently was quoted in news accounts as saying, "You can say anything you want in a debate and 80 million people hear it." When asked what happens if misstatements are corrected later in newspapers, Teeley replied, "So what? Maybe 200 people read it, or 2,000, or 200,000."

Ferraro said: "That cynical outlook of fooling most of the people most of the time must be the operating principle of Mr. Reagan and Mr. Bush. I don't know what they think they're doing when they say things that are plainly not true."